The nuttily deep Montana furniture system, designed by Peter J. Lassen, consists of 42 basic shelving units that are all 69.6 centimeters square (but come in four different depths). User-selected customization comes in choosing colors and finishes, and then you stack and build whatever you need. It's like Lego for interior designers.
The Montana system, or at least its roots, ain't new--Lassen conceived of an earlier version called the 6060 way back in 1974, and by '75 it was being "marketed by Fritz Hansen as a complement to Arne Jacobsen's designs." By '79 Lassen had created the Montana system proper, and he's been tinkering with it, and expanding into different lines of furniture, ever since. You can check out their stuff here.
If you're reading this from Berlin, Montana is sponsoring an on-site "Design Challenge" in your city on June 6th, as part of the International Design Festival Berlin.